The use of electrically self-heated resistors, hot wires, and hot films as thermal anemometer transducers is well known in the prior art. In such devices, a heated resistive element serves as a sensing element, and its physical geometry is used to define its spatial response to impinging airflow. The sensing transducer has a non-zero temperature coefficient of resistance and is maintained at feedback-controlled constant resistance. Transducer element pairs are used to determine direction and in some cases they are used to determine both speed and direction.
In a well-executed paired-element directional thermal anemometer, where speed and direction are both differentially read out, advantage is taken of common-mode rejection of unwanted spurious input signals such as those caused by rain, snow, drizzle, fog, salt accretion and the like. Examples of such anemometers are those made in accordance with the teachings of U.S. Pat. Nos. 4,279,147 and 4,794,795.
Earlier directional anemometers, such as those taught by U.S. Pat. Nos. 3,352,154, 3,900,819, 4,024,761, and 4,206,638, use single-ended thermal anemometer sensing element pairs where the velocity or speed component is taken as the sum of the element signals with respect to ground, and direction sign sense information is taken as the difference signal between the elements.
A somewhat different approach is disclosed by U.S. Pat. No. 3,498,127, wherein an orthogonal set of paired sensing elements is used to drive a cathode ray indicator displaying speed and direction.
Multi-component thermal anemometers are generally used outdoors in unattended or isolated locations and are openly exposed to the surrounding environment. In time they may accumulate dirt and their performance can deteriorate unless routine periodic cleaning is employed or naturally occurring rainfall cleanses the anemometer. Often, they are operated where varying amounts of oil fog and unburned hydrocarbon vapors are present, as at airports, offshore drilling platforms, near power plants, and near ship and naval vessel exhaust stacks. Regardless of configuration, and how the element pairs are used, dirt and oil vapor accumulation adversely affects anemometer calibration and, in the long term, measured component wind speed will decrease as anemometer sensitivity becomes impaired, while wind direction sensing is little affected. The differential paired-element anemometers ('147 and '795) provide a composite wind speed and direction component output and see little if any change in direction sensing precision since dirt and oil accumulation is random, is fairly uniform, and occurs as a common-mode phenomenon. Wind component magnitude output is decreased as the elements become coated, while element cosine response is virtually unchanged. The single-ended element pairs ('154 . . . '638) will see a more drastic change in sensitivity since their wind speed output is taken as the sum of two element signals for each component.
Historically, the weather services of the World use wind data in polar form (rho-theta) for wind speed and wind direction, and most mechanical and electromechanical wind sets are configured accordingly. World-wide gathering and transmission of wind data is also handled in polar form. For thermal anemometer planar wind measurement, two components are customarily sensed, indicating North-South and East-West wind, or headwind and crosswind. Generally, wind direction, theta, is determined from two thermal anemometer orthogonal components by computing the cotangent or tangent function, and the magnitude of the wind resultant, rho, is taken as the square root of the sum of the squares of the orthogonal components. Interfacing with position synchro repeater type indicators is facilitated by the use of three sets of element pairs oriented in a wye or delta configuration in the horizontal plane. Appropriate signal conditioning can provide simulated transmitter synchro signals by using suppressed carrier modulation. Alternatively, a Scott transformation can be used for coordinate conversion from modulated two-phase, sine and cosine, to modulated three-phase components.